1. Field of the Invention
This invention relates to a process cartridge, an electrophotographic apparatus and an image-forming method.
2. Related Background Art
Image-forming apparatus making use of an intermediate transfer belt are effective as full-color or multi-color image-forming electrophotographic apparatus in which a plurality of component-color toner images corresponding to full-color image information or multi-color image information are sequentially transferred to and superimposed on a transfer medium to output an image-formed material on which a full-color image or multi-color image has synthetically been reproduced.
Compared with conventional-technique full-color electrophotographic apparatus having an image-forming system in which toner images are transferred from a first image-bearing member electrophotographic photosensitive member to a second image-bearing member transfer medium fastened or attracted onto a transfer drum (e.g., the transfer system disclosed in Japanese Patent Application Laid-Open No. 63-301960), the full-color electrophotographic apparatus having an intermediate transfer belt have such an advantage that a great variety of second image-bearing members (transfer media) can be selected without regard to their width and length, including thin paper (40 g/m2 paper) and even thick paper (200 g/m2 paper) such as envelopes, post cards and labels. This is because the use of the intermediate transfer belt makes any processing or control (e.g., the transfer medium is held with a gripper, attracted, and made to have a curvature) unnecessary for the transfer medium.
In addition, the intermediate transfer member made in the shape of a belt enables effective utilization of space to make the apparatus main body compact and achieve cost reduction, because placement freedom in the image-forming apparatus can be greater than a case in which a rigid cylinder such as an intermediate transfer drum is used.
However, the intermediate transfer belt has a shorter lifetime than the electrophotographic-apparatus main body, and hence, under the existing conditions, it is indispensable to replace the belt in the middle of the use of apparatus. At the same time, it is necessary to install a waste-toner container in which the toner having remained on the intermediate transfer belt is to be collected, and to dispose of the toner thus collected.
In addition to these, it is necessary to replace many component parts such as an electrophotographic photosensitive member and a developing assembly, and the toner.
As a method of making these replacement parts into a unit or units (process cartridge) so as to be attached to or detached from the main body with ease, Japanese Patent Application Laid-Open No. 8-137181 discloses a technique in which the intermediate transfer belt and the electrophotographic photosensitive member are made into units independent of each other and are so placed as to be attached to or detached from the main body with ease.
However, such a means requires replacement units in a large number and makes user""s operation troublesome. Also, since the units are designed and placed independently of each other, a problem may arise such that the apparatus must be made large-sized and may involve a high cost.
As a means for solving such a problem, a technique is proposed in which the intermediate transfer belt and the electrophotographic photosensitive member as replacement parts are made into one unit (process cartridge) so as to be simultaneously attached to or detached from the main body and replaced. Such a technique is disclosed in, e.g., Japanese Patent Applications Laid-Open No. 6-110261, No. 10-177329 and No. 11-30944.
However, differently from a case in which the intermediate transfer belt is set at the time the apparatus main body is installed, such a technique in which the intermediate transfer belt and the electrophotographic photosensitive member are set up as one unit to provide a process cartridge which can be attached to or detached from the main body with ease, namely, the intermediate transfer belt and the electrophotographic photosensitive member are integrally set as a process cartridge, tends to cause some problems ascribable to the intermediate transfer belt.
One of such problems is a lowering of belt strength which is caused by a tension applied to the intermediate transfer belt.
Usually, in order for the intermediate transfer belt to be surely driven without slipping, a tension must be applied thereto, where the process cartridge in which the intermediate transfer belt and the electrophotographic photosensitive member are integrally supported stands stationary as the tension is kept applied for a long time until it is actually put into use. As a result, the intermediate transfer belt may cause a creep to increase in peripheral length.
The peripheral length having increased is absorbed to a certain extent by a tension roller having a stroke. However, the belt has already come to have a lower modulus of elasticity than an initial preset value, and may cause serious color misregistration when used actually, resulting in a lowering of the image quality level.
An intermediate transfer belt having a small elongation may also have such a great problem that it is cracked because of such tension and vibration at the time of distribution in the market.
The phenomenon of creep is known to accelerate in a high-temperature environment, and the process cartridge in which the intermediate transfer belt and the electrophotographic photosensitive member are integrally supported must be designed also taking account of such a high-temperature environment the process cartridge may encounter during its distribution.
In particular, with the progress in techniques for manufacturing image-forming apparatus in recent years, it has become possible for digital-development type printers and copying machines to develop minute and accurate latent images with a resolution of 600 dpi or more as exposure spots have been made smaller in size and more highly dense, and to obtain images with a high quality on account of, e.g., precise control of electric fields. As a result, a change in the modulus of elasticity and the surface roughness of the intermediate transfer belt, which have not come into question in the past, may greatly affect image quality, and it is an important subject to solve this problem.
In the above conventional techniques, however, measures have been taken to address difficulties the process cartridge, in which the intermediate transfer belt and the electrophotographic photosensitive member are integrally supported, may have when, e.g., it is left for a long time during transportation and storage, and it can not be said that any process cartridges have been designed taking such distribution channels into account. Hence, in the conventional process cartridge in which the intermediate transfer belt and the electrophotographic photosensitive member are integrally supported, there are problems such that a high management cost may result because of, e.g., strict safekeeping and restriction on a period of service, and that complaints from users may increase.
In addition, it is also an important subject to reduce running cost, and much more cost reduction must be achieved on the process cartridge in which the intermediate transfer belt and the electrophotographic photosensitive member are integrally supported and which come to be a replacement part. Also, in order to make handling easy, the miniaturization and the disposal of waste toner should fully be taken into consideration.
Thus, any process cartridge having perfectly solved the technical problems peculiar to the process cartridge in which the intermediate transfer belt and the electrophotographic photosensitive member are integrally supported and any electrophotographic photosensitive member having such a process cartridge have not been made available.
An object of the present invention is to provide a process cartridge which is easy to maintain, enables miniaturization and cost reduction of the apparatus, and affords good images even when having been transported or left over a long period of time; an electrophotographic apparatus having such a process cartridge; an image-forming method making use of such an electrophotographic apparatus; and an intermediate transfer belt for such a process cartridge.
The present inventors have made extensive studies on the achievement of simple maintenance, miniaturization and cost reduction of process cartridges and improvement in image quality. As a result, they have discovered that the intended object can be achieved by employing a process cartridge in which the intermediate transfer belt and the electrophotographic photosensitive member are integrally supported, further in combination with some measures.
More specifically, the present invention is a process cartridge which is detachably mountable to the main body of an electrophotographic apparatus; the process cartridge integrally comprising:
an electrophotographic photosensitive member for holding thereon a toner image;
an intermediate transfer belt having a contact zone coming into contact with the electrophotographic photosensitive member;
a primary-transfer means for primarily transferring the toner image from the electrophotographic photosensitive member to the intermediate transfer belt at the contact zone; and
a charge-providing means for providing the toner on the intermediate transfer belt with electric charges having a polarity reverse to a polarity the toner has at the time of the primary transfer and returning the toner on the intermediate transfer belt to the electrophotographic photosensitive member at the contact zone to clean the intermediate transfer belt,
the intermediate transfer belt having:
a modulus of elasticity of from 500 MPa to 4,000 MPa at elongation from 0.5% to 0.6% in the peripheral direction;
a breaking extension of from 5% to 850% in the peripheral direction; and
a surface roughness Ra of 1 xcexcm or less.
The present invention is also an electrophotographic apparatus comprising:
an electrophotographic photosensitive member for holding thereon a toner image;
a charging means for charging the electrophotographic photosensitive member electrostatically;
an exposure means for forming an electrostatic latent image on the electrophotographic photosensitive member thus charged by the charging means;
a developing means for developing the electrostatic latent image formed on the electrophotographic photosensitive member by the exposure means, to form a toner image on the electrophotographic photosensitive member;
an intermediate transfer belt having a contact zone coming into contact with the electrophotographic photosensitive member, through which the toner image is primarily transferred from the electrophotographic photosensitive member and thereafter the toner image having primarily been transferred is secondarily transferred to a transfer medium;
a primary-transfer means for primarily transferring the toner image from the electrophotographic photosensitive member to the intermediate transfer belt at the contact zone;
a charge-providing means for providing the toner on the intermediate transfer belt with electric charges having a polarity reverse to a polarity the toner has at the time of the primary transfer, in order to return the toner on the intermediate transfer belt to the electrophotographic photosensitive member at the contact zone to clean the intermediate transfer belt; and
an electrophotographic-photosensitive-member cleaning means for cleaning the electrophotographic photosensitive member;
the electrophotographic apparatus having a process cartridge in which the electrophotographic photosensitive member, the intermediate transfer belt, the primary-transfer means and the charge-providing means at least are integrally supported and detachably mountable on the main body of the electrophotographic apparatus; and
the intermediate transfer belt having:
a modulus of elasticity of from 500 MPa to 4,000 MPa at elongation from 0.5% to 0.6% in the peripheral direction;
a breaking extension of from 5% to 850% in the peripheral direction; and
a surface roughness Ra of 1 xcexcm or less.
The present invention is still also an image-forming method comprising the steps of:
a charging step of electrostatically charging an electrophotographic photosensitive member;
an exposure step of forming an electrostatic latent image on the electrophotographic photosensitive member thus charged in the charging step;
a developing step of developing the electrostatic latent image formed on the electrophotographic photosensitive member in the exposure step, to form a toner image on the electrophotographic photosensitive member;
a primary-transfer step of primarily transferring the toner image formed in the developing step from the electrophotographic photosensitive member to an intermediate transfer belt by a primary-transfer means, the intermediate transfer belt having a contact zone coming into contact with the electrophotographic photosensitive member;
a secondary-transfer means for secondarily transferring to a transfer medium the toner image having primarily been transferred in the primary-transfer step;
a charge-providing step of providing the toner on the intermediate transfer belt with electric charges by a charge-providing means, the electric charges having a polarity reverse to the polarity the toner has at the time of the primary transfer;
an intermediate-transfer-belt cleaning step of returning the toner on the intermediate transfer belt to the electrophotographic photosensitive member at the contact zone to clean the intermediate transfer belt; and
an electrophotographic-photosensitive-member cleaning step of cleaning the electrophotographic photosensitive member;
the image-forming method making use of an electrophotographic apparatus having a process cartridge in which the electrophotographic photosensitive member, the intermediate transfer belt, the primary-transfer means and the charge-providing means at least are integrally supported and detachably mountable to the main body of the electrophotographic apparatus; and
the intermediate transfer belt having:
a modulus of elasticity of from 500 MPa to 4,000 MPa at elongation from 0.5% to 0.6% in the peripheral direction;
a breaking extension of from 5% to 850% in the peripheral direction; and
a surface roughness Ra of 1 xcexcm or less.